Auxiliary Power Units (“APUs”) are a type of Gas Turbine Engine (“GTE”) commonly deployed onboard aircraft and utilized for any one of a number of non-propulsive purposes, such to provide power during Main Engine Start, to produce electricity by driving generators, to drive other components on the aircraft (e.g., pumps), and/or to provide cooling airflow. As do GTEs, generally, an APU typically includes an intake section, a compressor section, a combustion section, a turbine section, and an exhaust section arranged in flow series. During APU operation, airflow is received within the intake section, compressed within the compressor section, and then mixed with fuel and ignited within the combustion section. The combustive gases are then expanded through the turbine or turbines within the turbine section to drive the rotation thereof and thereby provide the power output of the APU. Finally, the combustive gases may be expelled from the APU through the exhaust section.
The performance of an APU is dependent upon a number of factors. One factor affecting APU performance is the range over which the compressor(s) included within the APU compressor section can operate without surge (referred to herein as the “surge-free operational range”). The surge-free operational range of the compressor may, in turn, be affected by the flow structure or conditions within the APU intake section and, specifically, with the intake section plenum. When the flow structure within the plenum of the APU intake section is highly unsteady or turbulent such that flow conditions vary significantly across different regions of the plenum and/or over time, the surge-free operational range at which the compressor can operate for prolonged periods of time may be undesirably reduced and the overall performance of the APU may be negatively impacted. In many cases, the flow structure within the APU intake plenum can be stabilized by imparting the plenum with a relatively large volume and/or by adding baffles to allow mixing and diffusion of the APU inlet airflow and to avoid impingement of high velocity or high Mach number airflow on any structures present within the plenum; however, in certain instances, such as when the APU is required to be highly compact, imparting the APU intake plenum with such a large volume may be impractical.
There thus exists an ongoing need to provide embodiments of an APU or other GTE (e.g., a turboshaft engine) wherein highly unsteady, time-dependent, or turbulent flow structures within the intake section can be stabilized to bring about improvements in the overall engine performance, even in instances wherein the GTE is relatively compact and the intake section plenum encompasses a relatively small volume. It would also be desirable to provide embodiments of a multimodal APU or other GTE operable in a number of different modes to provide additional functionality, while also having a relatively stable or predictable flow structure within the intake section plenum across all operational modes. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.